For example, Marmite Crumpets don't exist. You cannot buy them at the supermarket. To be clear: you can buy crumpets, you can buy marmite, you can buy butter; but you have to assemble them at home.
If you walk into a breakfast cafe, they will happily serve you sausage / egg / bacon / french toast / bubble / squeak (whatever that is). But no marmite crumpets. If you ask them to make it, they will give you a very strange look. It's not typically offered. It's something you just have to make at home.
It is unbuyable. Any tourist who comes to the UK to try a Marmite crumpet would need to bring a toaster or an oven with them, or quickly befriend a brit and hope that they have all the ingredients at home.
It's not a secret. You just can't have it.
*munches into crumpet thoughtfully, and salivates at the juicy savory delight, whilst staring at you pityingly and condescendingly*
Anyway, what's something that I could never experience unless I made it myself in your local?
Buckwheat kasha, you won't find it even in a Slavic restaurant. It is a simple dish of cooked buckwheat and milk, with sugar added if one desires. Such a simple breakfast dish is sold nowhere to my knowledge.
It sounds like chinese congee, but with wheat instead of rice
I've never had buckwheat that wouldn't have funky smell/aftertaste. It just weird all the time. Probably trying wrong brand or IDK. I'm slavic so my ancestors ate shitton of buckwheat, though it was almost non existent in my childhood. And now it's weird ingredient I'm scared of :-D